1. Field of the Invention
The invention relates to a process for the continuous production of aqueous polyurethane dispersions based on aliphatic and/or aromatic polyisocyanates.
2. Description of the Prior Art
From the prior art, essentially two batch processes are known for the industrial-scale production of aqueous polyurethane dispersions: the acetone process and the prepolymer mixing process. The production of dispersions based on aliphatic polyisocyanates is achieved relatively easily with both processes because of the moderate reactivity of the aliphatic polyisocyanates. The production of dispersions based on aromatic polyisocyanates generally takes place by the acetone process or the ketimine process, since both processes prevent excessive foaming in the dispersing step, caused by the isocyanate-water reaction. In WO 81/02894, another route is followed to obtain dispersions based on aromatic polyisocyanates (PIC). The reactive PIC groups are reacted with a blocking agent which can be split off again, so that foaming takes place only to a minor degree or not at all. Disadvantages of this process are—as in the ketimine process—on the one hand the additional blocking step and on the other hand the low molecular-weight blocking agent, which remains in the system or has to be removed in an additional step.
In EP-A 0 220 000, the production of aromatic dispersions by the prepolymer mixing process is described. However, the examples show that an external emulsifier is always required in the production process, which later remains in the coatings produced from the dispersions and brings the disadvantages known to the person skilled in the art. Furthermore, the neutralisation of the prepolymer takes place indirectly, i.e. the tert. amine needed to neutralise the carboxyl groups is added to the initial water. Direct neutralisation here leads to a marked reduction in NCO in the prepolymer, well below the theoretical NCO content. In addition, it is almost impossible to exchange the hydrazine used as a chain-extending amine for other amines that are reactive towards isocyanates. Finally, the dispersions obtained according to EP-A 0 220 000 possess a solids content of only 30 wt. %.
For the continuous production of aqueous polyurethane dispersions, a process is known from WO 98/41552, for example. In this process, dispersions are produced using mechanically driven dispersers, which are thus prone to breakdowns, and generally with the addition of external emulsifiers.
EP-A 0 303 907 also describes a continuous process in which aqueous polyurethane dispersions are produced. The preferred process provides for the metering of water into the higher-viscosity prepolymer at high speed, the inhomogeneous mixture being homogenised or the (pre)polymer dispersed in a mixing tube. A disadvantage of this process, which is very advantageous per se, is the increasingly deteriorating service life of the system when high-viscosity prepolymers are processed.
The object of the present invention is to provide a process for the continuous production of aqueous, aliphatic and aromatic polyurethane dispersions, which lacks the above-mentioned disadvantages. In particular, the process should also make it possible to produce polyurethane dispersions based on aromatic polyisocyanates, as well as to process high-viscosity prepolymers. Furthermore, the process should be distinguished by a high service life of the order of magnitude of 10 hours and more.